A number of methods have been in use publicly for a number of years for separating the impurities floating on a molten mass of metal from the molten mass itself. For the most part, the conventional techniques have drawbacks which render them undesirable to purify.
A dam is often used for this purpose. For a dam to function properly, it must protrude into the molten metal so that its lower edge is below the surface of the metal. As a result, it must be constructed of materials that can withstand the thermal environment or alternately, the dam must be water cooled. A dam that is able to withstand temperatures of this environment must be made of either ceramic or graphite if not H.sub.2 O cooled. Both of these materials are sources of contamination for many alloys. Both materials are subject to cracking of or reacting with the metal in this high temperature environment. As a result, dams made of these materials are often not acceptable for purifying the upper surface of a mass of molten metal.
Dams made of water-cooled copper may be used but they cause other problems. Since copper dams are cooled, they also have a tendency to cause the solidification of the metal around the dams. This action can make it difficult to achieve steady state material flow through the system. To counter this solidification, additional heat must be added in the neighborhood of the dam, thus allowing the area under the dam to remain molten. This technique generates extreme heat fluxes within the dam. As a result, the internal water passages of these dams scale up quickly and the dams are subject to a significant amount of thermally induced deformation. These effects contribute to high maintenance costs, short operating life and provide an opportunity for costly water leaks and low thermal efficiencies.
Another technique conventionally used is one known as oxide herding. This technique requires that a significant portion of the input power be used for herding of oxides. This can represent a process efficiency loss. It also puts a constraint on the patterning which may make some forms of process optimization impossible. Additionally, in the case of an "arc down" or momentary interruption of the herding heat source; the herding mechanism becomes immediately non-functional. This affords the opportunity for impurity flows to occur within the interval between the arc down and the moment that the torch or electron beam is restarted or that the metal solidifies. After restart, the slag may have moved to a location where recovery by the herding mechanism is impossible. Since at present technological levels arc downs do occur, this mechanism is not as efficient as it might be.
Because of these drawbacks of conventional techniques, a need exists for a more robust purification technique with the capability to make such a process more versatile for use and to reduce the cost of the purification while maintaining a high degree of purification of the metal.